"Jack Frost isn't nipping at your nose," the New York Daily News declared during a 1980s cold spell. "He's trying to tear it off."

So it was for much of this week. But it's not Jack Frost. It's the polar vortex, a pattern of high-level winds in the Arctic that circle the North Pole across North America, Europe and Asia. The vortex contains frigid Arctic air around the pole.

Likewise the jet stream, another pattern of high-altitude wind that circles the planet at lower latitudes, generally keeping warmer air to the south and colder air to the north.

When the polar vortex diminishes, cold air flows south, pushing the jet stream south, as well. That's why it was not only freezing here and in the Midwest, but very cold across the South.

Many climate-change deniers contend that the cold weather disproves global warming. Weather, however, isn't climate. It's what's happening at a given moment.

Many climate scientists say that the jet stream is slower partially because of diminishing polar ice that reflects sunlight, and its wind-generating energy, back to higher elevations. Instead, that heat is absorbed by exposed Arctic water, which is getting warmer

Studies over the last decade indicate that the more Arctic ice melts in the summer, the polar vortex weakens in winter. The result is broad fluctuations in weather rather than consistently seasonal heat or cold. Volatile is the new normal.